This part of ISO 17123 specifies field procedures to be adopted when determining and evaluating the precision (repeatability) of optical plumbing instruments and their ancillary equipment, when used in building and surveying measurements. This part of ISO 17123 is not applicable to optical plummets as a device in tribrachs or in surveying instruments. Primarily, these tests are intended to be field verifications of the suitability of a particular instrument for the immediate task at hand and to satisfy the requirements of other standards. They are not proposed as tests for acceptance or performance evaluations that are more comprehensive in nature. This part of ISO 17123 can be thought of as one of the first steps in the process of evaluating the uncertainty of a measurement (more specifically a measurand). The uncertainty of a result of a measurement is dependent on a number of factors. These include among others: repeatability, reproducibility (between-day repeatability) and a thorough assessment of all possible error sources, as prescribed by the ISO Guide to the expression of uncertainty in measurement (GUM). These field procedures have been developed specifically for in situ applications without the need for special ancillary equipment and are purposefully designed to minimize atmospheric influences and effects of imperfect adjustment of the optical plumbing instrument.